The inventors have developed an ignition plug disclosed in Japanese patent application publication No. sho 62-11471. In the ignition plug, an ignition groove is provided on one side of a ground electrode opposite a center electrode in such a way that a gas mixture in the groove may be ignited by ignition seeds in the spark gap so as to prompt combustion in the combustion chamber.
However, it has become clear that with this system, no more powerful initial ignition can be hoped for even if more than one ground electrode, each provided with the ignition groove, would be disposed around the center electrode. It is because with this old system, such sparks as leading to the ignition of gas mixture in the groove would not appear in the respective spark gaps at the same moment, but only in any one of them every combustion cycle.